Such a capacitive micromachined ultrasonic transducer cell, probe and imaging system are disclosed in EP1944070.
These products are used for medical diagnostic scans.
CMUT transducers are tens of micrometer size diaphragm-like cells that usually comprise two electrodes opposing each other. For transmission the capacitive charge applied to the electrodes is modulated to vibrate/move the diaphragm (membrane) of the device and thereby transmit a sound wave. Objects placed in the way of the sound wave propagation reflect the wave. The reflected sound wave causes vibrations of the membrane, modulating the capacitance between two electrodes of the CMUT transducer, thereby generating an electric signal. This signal is representative for the reflected sound wave hitting the membrane.
The CMUT cells are arranged in an array for high-frequency beam-forming.
For good performance, CMUTs require a high bias voltage (e.g. 100V) applied between the electrodes.
In the cases, where 3D imaging is not feasible (e.g. due to cost reasons, limited available area or required frame rate), a bi-plane 2D imaging may be applied.
In bi-plane imaging the array of CMUT's is divided in two groups of transducer elements arranged substantially along two directions, wherein each CMUT element can be activated within both groups. In the prior art this is realized by connecting all top electrodes of the elements in array to a metallized pattern having a main orientation along the x-direction and all bottom electrodes to a metallization pattern with main orientation along the y-direction. A DC source is supplied to each element of the group through an RC-filter (also known as bias-T filter). This bias T-filter separates the AC-component of the signal from the DC-component so that the ASIC technology for processing the detection signals of the probe only has to cope with the AC-component of the signal and is prevented from exposure to high voltage.
The bias-T filters in the array comprise a discrete high-voltage coupling capacitor and a discrete high impedance resistor for each interconnected element. In case of “N” elements in the group, “N” discrete high-voltage capacitors and the same amount of discrete resistors have to be provided. Due to the relatively large dimensions of these high-voltage components the application of the T-bias filters in CMUT arrays eliminates gained benefits of miniaturization and can be hardly implemented in ultrasound catheters, for example. Further, parasitic effects of these discrete components may limit the performance of the array.